The flange face finish concept refers to the type of roughness of a flange face. Some roughness is necessary to ensure that the flange face mates with the gaskets and the companion flange perfectly. The common flange face finishes are the stock, the concentric serrated, the spiral serrated and the smooth types.
FLANGE FINISH TYPES
Steel flanges are available with four basic face finish. The common objective of any type of flange face finish is to create the desired roughness on the face of a flange to ensure a strong match between the flange, the gasket, and the mating flange and thus provide a high-quality sealing.
Source: Officine Orsi, Italy
The stock finish is the most widespread type of finish as it suits the large majority of applications. The pressure embeds the soft face of the gasket into the flange finish and results in the formation of a good seal due to the friction existing between the contacting parts.
As the mating flanges are bolted together, gaskets get “squeezed” into the flange face surface and create a very tight seal.
A stock finish face is manufactured using a phonographic spiral groove featuring a 1.6mm radius round-nose tool with a depth of 0.15mm and a feed-rate of 0.8mm per revolution. The resulting “Ra” value (AARH) for the surface ranges from 125µinch to 500 µinch (125 µm to 12.5 µm).
Spiral serrated finish is a phonographic spiral groove type that differs from the stock finish as the groove is crafted by a 90 degrees tool (instead of a round nosed one) that creates a “V” geometry with a 45-degree serration angle.
A serrated finish, concentric or spiral, has from 30 to 55 grooves per inch and a roughness between 125 to 250 µinch.
The concentric serrated flange finish features concentric grooves instead of spirals.
The grooves are crafted by the same 90-degree tool used for the spiral serrated finish, but the serrations have an even design on the face of the flange. To have concentric grooves, the tool has a feed rate of 0.039mm per revolution and a depth of 0.079mm.
SMOOTH FINISH OR FACE
Flanges with a smooth finish do not show visible tool markings at naked eye.
This type of flange finish is used with metal-facing gaskets such as the jacketed type. As per the stock finish, this is achieved by having the contact surface machined with a continuous spiral groove generated by a 0.8mm radius round-nosed tool at a feed rate of 0.3mm per revolution with a depth of 0.05mm (that creates a roughness between Ra 3.2 and 6.3 micrometers, i.e. 125 – 250 microinches).
The cold water finishes appear shiny to the naked eye and very smooth. The AARH value for these surfaces ranges between 85 µinch to 100 µinch. They are used with metal to metal seals (no gasket).
WHAT IS AARH (ARITHMETIC AVERAGE ROUGHNESS HEIGHT)?
The term AARH (“arithmetic average roughness height”) refers to the flange face smoothness/roughness. The average arithmetic roughness height values are very important during the selection of flanges and gasket materials. Higher the “Ra” values depict a more rough surface, while lower values represent the smoother surface.
Every material possesses a surface roughness and sometimes surfaces are finished deliberately to have a specific roughness (small or bigger).
The “Arithmetic Average Roughness Height” (AARH) is the common indicator to measure the roughness of a surface, and it is the average height of the irregularities on the metal surface, from the mean line as shown in the following figure.
The symbol Y1 to Y8 represent the peak heights which are measured from the mean line.
Arithmetic Average Roughness Height is usually measured in micro-inches and denoted by symbol “Ra”.
There are various standards for the roughness of surfaces, set according to their type of application. The equipment used to measure the surface roughness is the so-called “profilometer” (which are available in the contact and non-contact types).
In contact type profilometer the roughness is measured by moving the material under the profilometer stylus. However, modern equipment features non-contact measurements, leveraging the optical and ultrasonic technologies.
ASME/ANSI defined specific roughness standards for the flanges, as the flange face finish plays a pivotal role in gasket’s reliability and service life.
According to the ASME/ANSI specifications, the serrated, spiral serrated, and concentric flange face finish should have an average roughness of 125 µinch to 250 µinch (3.2 µm to 6.3 µm).
The tool used to imprint a rough finish on the flange should have a radius of 0.06 inch (1.5mm) or larger. The groove density on the flange face should be from 45 grooves per inch to 55 grooves per inch (1.8 grooves/ mm. to 2.2 grooves/ mm.).
These are the standards for semi-metallic and nonmetallic gaskets. If the average roughness of flange face is not according to the described standards, the contacting surfaces would not properly seal and the flanged joint may wear after some time working under pressure (resulting in loss of bolt joint tightness and a possible leakage).
The soft nonmetallic materials such as PTFE may be used for more comfortable facing and better creep resistance.
ALLOWED AARH IMPERFECTIONS
The sealing performance of the flanges’ gaskets depends on the AARH, the flange dimensions and the pressure of the stud bolts. According to ASME, the adjacent imperfections should be separated by a distance of at least 4 times the maximum radial projection.
The radial projection can be evaluated by subtracting the inner radius from the outer radius. The serrations shall be at the same level, and the protrusion above them is not permitted. It can cause the adjacent serrations to lose hold of the gasket material and may result in wears and leakages.